The present invention relates to a new process for preparing vitamin A and its intermediates; it also relates to a new process for preparing intermediates of vitamin E and carotenoids.
Vitamins A and E have been prepared chemically for a long time. The preparation processes are numerous and varied. A relatively exhaustive account of the processes used industrially can be found in an article published by J. Paust in Pure & Appl. Chem., Vol. 63(1):45-58, (1991).
In this article, Paust described various industrial techniques for carrying out the condensation of various polyenic units in order to arrive at vitamin A. This condensation is performed either via a sulphone (U.S. Pat. No. DE 2,202,689) or via an acetylenic compound which is used in a Grignard reaction (article which appeared in Pure & Appl. Chem. 47:183 (1976)) or finally via a Wittig reaction involving a phosphorus ylide (U.S. Pat. Nos. DE 1,046,046 and DE 1,026,475).
Each of these three techniques for condensing various polyenic units uses, as starting material, a relatively expensive intermediate which, on its own, will enable the condensation to be performed in each of the processes described, thus making this step particularly expensive in the case of all the processes described. Further, because the starting intermediates are expensive, a step for recycling or removing the by-products formed is essential in all these processes. Phosphonium is converted to phosphine oxide which has to be reduced to phosphine; the sulphonic derivative produces, at the end of the reaction a phenyl sulphinate which has to be recycled; during the reaction, the acetylenic derivative consumes magnesium which generates salts that have to be removed.
We therefore sought a new process for preparing vitamins A and E, which makes it possible to dispense with these sulphones, acetylenic compounds and phosphorous ylides.